A communication system is formed, at a minimum, of a transmitter and a receiver interconnected by a communication channel. Communication signals generated by the transmitter are transmitted upon the communication channel to be received by the receiver.
A radio communication system is a type of communication system in which the communication channel is formed of a radio frequency channel. A radio communication system is advantageous for the reason that a fixed, or hardwired, connection is not required to form the communication channel extending between the transmitter and receiver. Communication can be effectuated between remotely-positioned transmitters and receivers without the need to form the hardwired or other fixed connections therebetween.
A cellular communication system is a type of radio communication system. When the infrastructure, hereinafter referred to as the network, of the cellular communication system is installed in a geographical area, a subscriber to the cellular system is generally able to communicate telephonically in the system when positioned at any location within the geographical area encompassed by the cellular network.
Technological advancements and economies of scale have contributed to lowered costs of communicating pursuant to a cellular communication system. Concomitant with the decreased costs of communicating pursuant to a cellular communication system has been an increase in usage of such systems. In some instances, conventional cellular communication systems have been operated at their maximum capacities. When operated at their maximum capacities, access is sometimes denied to additional users attempting to communicate pursuant to such systems. Ongoing calls are sometimes also adversely affected.
To avoid capacity problems and to permit an increase in the number of users permitted to utilize a cellular communication system, attempts have been made to increase the communication capabilities of such systems. To increase communication capacities, some conventional, cellular communication systems using conventional, analog technologies have been converted to digital, cellular communications systems which utilize digital coding and modulation technologies. Other radiotelephonic, and other communication systems, have been similarly converted.
Because digital communication systems generally utilize the radio frequency transmission channels upon which the communication signals are transmitted more efficiently, increased numbers of communication signals can be transmitted upon the radio frequency channels allocated for such communication systems.
By digitizing an information signal, which is modulated to form a communication signal, signal redundancies can be removed out of the information signal without affecting the amount of information transmitted in a communication signal formed therefrom. Also, once an information signal is digitized, a communication signal formed therefrom can be transmitted in discrete, discontinuous bursts. Two or more communication signals can thereby be multiplexed together and transmitted sequentially upon a single frequency channel. A twofold, or greater, increase in capacity is thereby possible when the communication system is converted into a digital communication system.
The communication signal transmitted upon a radio frequency channel is susceptible to scattering, diffraction, reflection, and attenuation. Signal reflection of the transmitted signal causes the signal actually received by a receiver to be the summation of signal components transmitted by the transmitter by way of, and some instances, many different paths, in addition to a direct, line-of-sight path.
The communication channel is sometimes referred to as a "multi-path channel," as the signal actually received by the receiver is the summation of a plurality of signal components transmitted to the receiver on a plurality of different paths. Values of the signal components transmitted upon the multiple numbers of paths are dependent, in part, upon their relative phases. Therefore, the value of the summation of the plurality of signal components received by the receiver is dependent upon the position at which the receiver is located when the signal is received. The receiver might be positioned such that signals transmitted on the plurality of signal paths add together destructively. Signal "fading" occurs when the signals add together destructively, and fading "dips" or "nulls" occur when the summation of the received signals results in cancellation when such destructive addition makes difficult, or prevents, accurate determination of the informational content of the received signal.
Because fading deleteriously effects the quality of communications, attempts are sometimes made to mitigate the effects of fading. The deleterious effects of fading are particularly problematical in a cellular communication system when a subscriber unit operable to communicate therein is maintained in a stationary, or slowly moving, position. If positioned at an area in which a fading dip is significant, a significant amount of sequential information might be lost.
More particularly, various kinds of diversity are created at selected locations of the digital communication system to mitigate the effects of the multi-path fading. Time diversity, frequency diversity, receiver space diversity, transmission space diversity, and polarization diversity are all types of diversity which can be created to mitigate the effects of multi-path fading.
When time diversity is created, signal bits of an informational signal are spread out, or interleaved together with other signal bits, over time, thereby to spread the bits over a time period. When interleaved or otherwise spread out over time, the likelihood that all of the bits are received at a receiver at a fading null is reduced. To be effective, the bit spreading requires that the fading nulls of the multi-path channel not last for significant time periods. If the nulls last for lengthy time periods, spreading out of the bits of the informational signal does not create diversity of levels effective to mitigate the effects of fading.
Receiver space diversity is also sometimes created. To create receiver space diversity, two or more receiver antennas are positioned at two or more spaced apart positions, or at two or more different angles. The creation of receiver space diversity requires at least minimum physical separation distances between the receiver antennas. Such minimum separation distances cannot be provided at a subscriber unit operable in a cellular communication system due to the small size of the subscriber unit. Also, the conventional need for redundant receiver circuitry portion for each of the two or more receiver antennas is size-prohibitive in a subscriber unit which must be of minimal dimensions. Therefore, receiver space diversity sometimes cannot be utilized to create necessary levels of diversity to mitigate the effects of fading.
Frequency diversity is also sometimes created to minimize the effects of multi-path fading. Frequency hopping, i.e., transmitting bursts of a communication signal on carriers of different frequencies, spreads the communication signal over various frequencies. In a digital cellular communication system, such as the Group Special Mobile (GSM) communication system, an adequate level of frequency diversity is sometimes unable to be created as sometimes only a limited number, as few as, for instance, two, different carriers are available to transmit communication signals between a radio base station and subscriber unit. As multi-path fading is generally frequency-selective, the transmission of bursts of the communication signal on a different frequency carriers provides a diversity effect. However, the number of different frequency carriers upon which the bursts of the communication signal can be transmitted are sometimes limited. Such limitations limit the amount of frequency diversity which can be created.
Also, when successive carriers upon which successive bursts of a communication signal are transmitted are of similar fading characteristics, little frequency diversity is created. The coherence bandwidth is a frequency range which exhibits similar fading characteristics. When successive bursts of communication signals are transmitted upon carriers which are within the coherence bandwidth, little frequency diversity is created by such frequency hopping. If successive bursts of the communication signal are not transmitted on carriers within the same coherence bandwidth, communication quality degradation occurring as a result of multi-path fading is of less of a problem. Appropriate selection of the carriers upon which to transmit successive bursts of the communication signal would therefore advantageously better overcome the deleterious effects of multi-path fading.
Transmission space diversity is sometimes also created. To create transmission space diversity, two or more transmitting antennas are positioned at spaced-apart positions. In one manner of creating transmission space diversity, the same information is transmitted by each of the two or more transmitting antennas, but the information is transmitted at offset times. In another manner by which transmission space diversity is created, bursts of the communication signal are transmitted at only one antenna at a time, but shifting between the antennas occurs so that the bursts are transmitted sequentially to different ones of the antennas. Such a manner of creating the transmission space diversity is referred to as antenna hopping. Existing circuitry for creating transmission space diversity, however, does not typically provide for complete freedom of selection of antennas, particularly in instances in which statically-tuned transmitter elements are utilized to modulate the communication signal. Polarization diversity is also sometimes created to minimize the effects of multi-path fading. Polarization hopping, i.e., transmitting bursts of a communication signal in different polarizations, permits the bursts of a communication signal to be transmitted according to different polarizations.
While the creation of receiver space diversity is impractical in the mobile subscriber units of a cellular communication system and similarly in some other communication systems, and while the creation of time diversity is sometimes unable to overcome the effects of multi-path fading, frequency, polarization, and transmission space diversity can be created at a base site of a cellular communication system to overcome the effects of multi-path fading. Other communication devices of other communication systems which are susceptible to multi-path fading similarly can create frequency and transmission space diversity to overcome the deleterious effects of fading. Circuitry and methodology for a communication device which permits greater freedom in the creation of transmission space diversity and frequency diversity would be advantageous.
It is in light of this background information related to the creation of signal diversity to overcome the effects of multi-path fading that the significant improvements of the present invention have evolved.